The present invention relates generally to an electrophotographic printing machine, and more particularly concerns an optical system utilizing a half-tone screen.
In the process of electrophotographic printing, an electrostatic latent image is recorded on a photoconductive member and developed with toner particles. Generally, a sheet of support material is brought into contact with the toner powder image and the particles are transferred thereto, in image configuration. Thereafter, the toner particles are permanently affixed to the sheet of support material to form a copy of the original document. The photoconductive member is initially sensitized by charging the surface thereof substantially uniformly. After the photoconductive member is charged, a light image of the original document is projected thereon. This light image selectively dissipates the charge on the photoconductive member to record an electrostatic latent image thereon.
Multi-color electrophotographic printing is similar to black and white printing. In multi-color printing, the process is repeated a plurality of cycles, each cycle being for discrete color contained in the original document. The light image is filtered to record an electrostatic latent image on the photoconductive member corresponding to a single color. A plurality of different single color light images are formed. Each single color electrostatic latent image is developed with toner particles complementary in color to the color of the filtered light image. These toner particles are then transferred to the sheet of support material in superimposed registration with one another to form a multi-color copy corresponding to the original document. Thereafter, the multi-layered toner powder image is permanently affixed to the sheet of support material by the application of heat thereto.
An electrophotographic printing machine may reproduce a functional or pictorial document. Functional documents visually do not have subtle variations of tone or color. Such documents typically contain line information and are typed sheets, graphs, charts, lines, etc. A pictorial document may be reproduced obtained by utilizing a half-tone screen. The screen produces tone graduations by forming half-tone dots or lines of varying size. In the highlight regions, the half-tone pattern may comprise narrow lines or small dots. The lines or dots increase in width and size throughout the intermediate shades until they merge together at the shadow end. Thus, there will generally be complete whiteness at the high light end and nearly solid blackness at the shadow end of the tonal scale.
The process of screening may be carried out generally in either of two ways. One approach, generally termed multiplicative in the art, is to transmit the light image of the original document directly through the screen to expose the charged photoconductive member therewith. The screen modulates or finely divides the light image to form a half-tone image. A second approach forms a light image of a screen pattern and irradiates the charged photoconductive surface therewith. Prior to or subsequent to the formation of the screen light image, a light image of the original document is projected onto the charged portion of the photoconductive member. The light image of the original document and the screen light image are superimposed on the photoconductive member. Thus, the resultant electrostatic latent image is finely divided by the screen pattern. In sequential screening, as this latter approach is termed in the art, the screen and light source associated therewith are disposed prior to or subsequent to the projection of the light image of the original document onto the photoconductive member. In multi-color electrophotographic printing, the problem of Morie' patterns must be overcome. Frequently, this is achieved by rotating the screen between successive single color light images to minimize this effect.
Many patents exemplify the art of screening. U.S. Pat. Nos. 2,598,732; 3,535,036; 3,121,010; 3,493,381; 3,776,633; and 3,809,555 all teach various screening techniques. Of particular note are U.S. Pat. Nos. 3,535,036 and 3,540,806, both having been issued to Starkweather in 1970. The Starkweather patents disclose the use of timed light flashes through a light baffle having a screen onto a photoreceptor surface to form a half-tone light pattern thereon. The screen may be located before or after the exposure station. U.S. Patent application Ser. No. 701,445 filed June 30, 1976, now abandoned, discloses a screening system wherein a light source illuminating the screen is flashed sequentially as a function of the photoconductive position. This signal actuates a lamp illuminating the screen member periodically. In this manner, the effect of speed errors in the movement of the photoconductive surface are minimized. However, none of the foregoing approaches appear to correct for Morie' effects as well as speed variations.
Accordingly, it is a primary object of the present invention to improve the screening system employed in electrophotographic printing machines by controlling screen illumination as a function of the movement of the photoconductive member with respect to the spacing between adjacent rows of dots on the screen.